An agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives was shot during an operation Thursday, the agency said.
Agents from the Chicago Field Division were "ambushed" while conducting an undercover operation in Gary, Indiana, ATF Agent Brendan Iber said in a news conference.
"One of our agents was shot multiple times and was transported to the Methodist Hospital here at Northlake campus," Iber said.
The wounded agent is in critical but stable condition, Iber added.
"Due to the fact that it's an ongoing investigation at this time, I cannot release any more information, but I would want to thank the law enforcement community, the hospital staff and the public for the outpouring of support we've received so far."
The Chicago Police Department expressed solidarity with the agency after the news broke.
"Thoughts and prayers for the @ATFHQ Chicago Field Office agent who was shot and critically wounded in Gary, Indiana," the department said in a tweet.
"ATF agents are working with us every hour of every day to reduce gun violence in Chicago and it's beyond a partnership, it's one team, one fight."
The FBI's gang response investigative team responded to the scene to assist ATF, FBI Indianapolis Public Affairs Specialist Chris Bavender said. The Indiana State Police are also investigating.
Elsewhere in Gary, more people were injured in a car crash after the shooting. Amid reports that the crash was related to the shooting, ISP released a statement detailing the wreck.
"This afternoon at approximately 12 p.m., the Lake County Sheriff's Department was involved in a pursuit in Gary that resulted in multiple people being injured. Preliminary investigation indicates that a white Kia Optima fled the scene of a traffic stop at 7th and Tennessee in Gary. As officers pursued the vehicle, it fled southbound on Broadway. As it entered the intersection of 21st Ave at a high rate of speed, it struck two other vehicles," the statement said.
"The identities of the drivers and passengers and all who were transported for treatment is still under investigation. There were earlier reports that this crash was related to an officer involved shooting. That shooting is being investigated by the Indiana State Police," the statement continued. "The reason the driver fled which then resulted in the subsequent pursuit and crash is still under investigation."